1. Field of the Invention
The present invention relates to a technology for encoding/decoding an image, in particular, a color image.
2. Description of the Prior Art
Recently, color documents are rapidly widespread, and a data amount of documents is increasing several times the data amount of the past. Therefore, in order to reduce document input and output times, there is requested a color image compression coding system for compressing color image data at a high speed with a high compression ratio.
There are available a color image expression system in which image data is separated per color element such as K (black), Y (yellow), M (magenta), C (cyan) and expressed (this system will hereinafter be referred to as a field-sequential system) and a color image expression system in which image data is not separated per color element and expressed (this system will hereinafter be referred to as a dot-sequential system). These examples will be described with reference to memory areas 701 and 700 of FIG. 15. In the example of the field-sequential system expressed by the memory area 700, R (red) signal data, G (green) signal data and B (blue) signal data are memorized separately. In the example of the dot-sequential system expressed by the memory area 701, an R pixel value, a set of a G pixel value and a B pixel value expressing pixels are memorized at memory locations corresponding to the pixel positions of image. Data expressed by the dot-sequential system is read out as it is and processed when it is displayed on a CRT (cathode-ray tube) in a dot-sequential system. According to the dot-sequential system, an image can be drawn at a high speed and the dot-sequential system is widely used on a computer. On the other hand, an electrostatic photography color printer executes an electrostatic photography process at every element. Accordingly, the field-sequential system is widely used in this kind of color printer. For this reason, it is frequently observed that an image of a dot-sequential system handled by the computer is outputted by a field-sequential system printer.
A conventional example in which a dot-sequential system image is outputted by a field-sequential system printer will be described below. In the following description, a color image comprises four colors of K, Y, M, and C.